Piedmont Virginia Community College
Fall 2018 | W 7 - 9:45 PM
Syllabus
Documentation Guidelines
Fall 2018 Open Lab Hours
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MEC155 - Mechanisms:
Studies the purpose and actions of cams, gear trains, levers, and other mechanical devices used to transmit control. Focuses on motions, linkages, velocities, and acceleration of points within a link mechanism; layout method for designing cams and gear grain.
Required Materials:
Mechanisms and Mechanical Devices Sourcebook, 5th Edition - Neil Sclater
Resources:
Textbook
We previewed the topics for the coming semester and what sort of mechanical devices we are going to take a deeper look at and utilize them in developing mechanical systems. We then spent the majority of the time playing with OpenSCAD and the Universal Laser Cutter. We built a simple adjustable compass for making all the circles we need. Everyone designed their shapes in parametric CAD and sent the SVG files over to the laser cutter to be cut out of .2" plywood. Be careful exporting to SVG - OpenSCAD is unitless and can import differently in different programs.
Design Files:
Quick Compass - OpenSCAD
Quick Compass - Thingiverse
Help:
OpenSCAD Cheat Sheet
Inkscape to Universal Control Panel - Youtube
VLS Laser Cutter User Manual
For next class:
Finish your compass!
Photo/Media Release
Safety Guidelines
Safety Guideline Agreement
We looked at the basics of machine efficiency and mechanical advantage. We had time to finish up our compass designs, add something to seal the wood, and wax to reduce friction and add a layer of protection. The basic mechanisms we began to discover included inclined plane, pulleys, screw jack, levers, and linkages. We can refer to pages 2-8 of the Sclater text. We paired up to create a simple application of each of these basic mechanisms. The requirements were:
Using OpenSCAD, design an application that can be laser cut on wood of your particular mechanism
Calculate the efficiency and mechanical advantage of your theoretical system and then the actual.
Share your application, design, and results with the class
Resources:
8.29-Class Notes
Linkage Library - OpenSCAD
Pulley Library - OpenSCAD
We spent this class laser cutting our OpenSCAD designs and building a testing rig to build a actual mechanical advantage and compare it to our theoretical. We presented our design, testing rig, results, and conclusions based on how we might improve the design or what went wrong through testing. Next class we will focus on more advanced mechanisms and finish our presentations.
We finished sharing our projects from the previous class and discussing our design strategies and testing procedures. We looked at what worked for us and what went wrong. We spent a portion of the class reviewing OpenSCAD and building some parametric designs to further our skillset. Our final goal is to work on building an adjustable pantograph. The pantograph must:
be adjustable to 3 different scale sizes
able to be used with a 2D medium, like paper, whiteboard, cardboard, etc...
have a fixed point, mounted
have a supporting rolling system so it moves fluidly when scaling objects manually
the entire mechanism must be coplanar
Resources:
Inclined Plane Example - OpenSCAD
Linkage Example - OpenSCAD
Linkage Modules Example - OpenSCAD
Student Project Examples - .zip
We spent the period discussing our thoughts and design decisions on building our pantographs. Many people found some great resources online to help further build a workable model. Make sure you test each of your scales with a simple drawing to find your relative error. We looked at designing these linkages and some strategies for creating modules that offer quick iterative troubleshooting. The next project involves developing a gear generator in OpenSCAD. You must use for loops to create parametric design. Pages 10-13 in the Sclater text describe gear geometry and tooth design.
Gear Generator:
Gear tooth geometry is parametric
Gear Ratio is adjustable
Gear has an outer radius
Use a for loop to create patterns
Custom Linkage:
Pick a linkage from pages 6-9
Model your linkage in 3D, use cylinder
Be prepared to create animations on your 3D assembly
Resources:
for loop - OpenSCAD
Build a Pantograph - Peter Lewis
MCAD Library - OpenSCAD
Using Gears MCAD - OpenSCAD
We spent a portion of the class working through designing custom gears and a drive system on the laser cutter. We looked through designing gears in OpenSCAD for the laser cutter and building a simple design using a DC motor. Next class we will finish our gears and add a motor driver to test our gears set.
Resources:
Design of Involute Gears - Fellows
Involute Gear Math - KHG
This class opened up H-Bridges and simple transister circuits to build motor drivers to help us test our DC motors with our gear designs. We will share these next class.
Resources:
L293D Basic Setup - Schematic
TIP120 Basic Setup - Schematic
L293D Datasheet
TIP120 Datasheet
Today we worked through the remainder of the electronics for basic motor driving. We shared our gear designs and completion. The next project will include a variety of mechanisms and require them to work together. You will be designing:
Belt Grinder/Sander
Use a minimum sanding belt at 1" x 10"
Contain a DC Motor
DC Motor geared to specific RPM at drive wheel
Contain a variable speed control using electronics, gear ratios, or pulleys
The table must be adjustable using a hinge or trunnion system
The belt tension must be adjustable
The lateral tracking of the belt must be adjustable
Resources:
608 Skate Bearing - Datasheet
Building Belt Grinder - Jeremy Schmidt
2x42 Belt Grinder Build - John Heisz
Various Plans for 2x72 Belt Grinders - CNCCookbook
2x72 DIY Belt Grinder - DCKnives
We continued our belt grinder progress, welding, cutting, designing... This project will be due at the end ofthe semester. We reviewed the later mechanisms in the text and compared them to our intial mechanisms.
We utilized this class period for labs and working on our projects.
We looked at Chapter 2 in the Mechanisms book diving into linear motion, control drives, and building numerical control systems.
We discussed differences in different types of linear motion and systems and compared their affordances and drawbacks. We worked on our belt grinder project and introduced the final project, an interactive mechanisms board. Please select a mechanism for class
Requirements:
Choose one Advanced Mechanism to Display
Move with Electronic Control
Interactive with Human Input
Educational Engagement including part labels, simple math, and explanation
Fit on a 2' x 1' rectangle area
Mount on .75" plywood
Presentations:
12/12 @ 5:15 PM in M833
Describe your design & Solution
Share digital & physical designs
Pictures & explanations of iterations
Math behind your mechanism
Choice of interactivity
Mounting Strategy
Demo of your working project
We will begin presentations at 5:30 PM, present until 6:30 PM, then cleanup lab space. Your presentation should not exceed 10 minutes.